A breech mechanism having a mushroom shaped obturator is, for example, disclosed in U.S. Pat. No. 4,308,785. In this known arrangement a slidable breech block is controlled via a pivotally mounted lever during the recoil movement of the gun barrel. In this known arrangement a mushroom-head obturator is moved via guide rollers disposed in the breech block, first in the direction of the gun barrel axis away from the end of the gun barrel rear end and then transversely with respect to the first movement. For purposes of closing the rear end of the gun barrel the movement is in the opposite direction. The breech block is first moved laterally away from the gun barrel axis in order to permit the afore-described stepwise movement of the mushroomhead.
German Pat. No. 1,870,778 also discloses a wedge-shaped breech block mechanism, the breech block of which is controlled by the recoil of the gun barrel. Also in this case the mushroomhead obturator must, in order to open the breech, be slidably moved axially away from the gun barrel end. Thereafter this wedge-shaped breech block is moved transversely with respect to the gun barrel axis. In order to achieve this bidirectional motion, a skewed toothed pinion is mounted on the mushroom-head support and a skewed toothed gear rack is mounted on the breech block. These known breech mechanisms are constructionally complex; they require at all times a two-step operation. This in turn inhibits the safety of the arrangement with respect to misfiring or jamming. Consequently, these known arrangements have a limited firing cadence capability and are found to be unsuitable when very high gas pressures are used.